ABSTRACT Cardiac abnormalities including arrhythmia and conduction delays are the second leading cause of mortality in patients with myotonic dystrophy type 1 (DM1). Despite the significant role of heart dysfunction in the prognosis of DM1 patients, its molecular mechanisms remain largely unexplored. Recent studies have shown that the cardiac voltage-gated sodium channel gene SCN5A (sodium voltage-gated channel alpha subunit 5, Nav1.5) undergo a different alternative splicing pattern in DM1 patients compared to the general population-- a shift in the expression of the mutually exclusive exons 6B toward 6A. Exon 6B is predominantly expressed in adults ('adult' exon), whereas exon 6A predominates in fetal development ('fetal' exon). DM1 heart tissue expresses varying levels of the fetal Nav1.5 isoform in addition to the adult isoform. Previous work done by Onkal et al. has demonstrated that the fetal Nav1.5 isoform possesses different electrophysiological properties. Therefore, we hypothesize that the change in Scn5a mRNA isoforms in DM1 contributes to arrhythmias and conduction delays prevalent in this disease. The sponsor's lab has generated a heart-specific tetracycline-inducible DM1 mouse model for DM1 (TREDT960I/MHCrtTA). These animals have an increased expression of the Scn5a fetal exon as well as electrocardiography (ECG) abnormalities. In Aim 1, I will use AAV with antisense sequences targeted to the Scn5a fetal exon to redirect splicing to predominantly the adult exon to test amelioration of the cardiac phenotypes. Furthermore, with the goal of determining the contributions of fetal Nav1.5 to aberrant cardiac phenotypes in DM1, we used CRISPR/Cas9 to generate mouse lines in which the Scn5a adult exon has been deleted in fertilized eggs, forcing expression of only the fetal isoform from the knockout allele. Preliminary ECG analysis on homozygous and heterozygous Scn5a adult exon knockout mice demonstrate age-dependent abnormalities in both genotypes compared to wild type littermates. In Aim 2, these CRISPR knockout mice, in addition to the DM1 heart model, will be subjected to atrial pacing and challenged with a sodium channel blocker to reveal subtle phenotypes. In Aim 3, I will be using isoform-specific antibodies to determine the atrioventricular and cellular localization of the fetal and adult isoforms in our mice using immunofluorescence imaging. The outcome of this project will be to: (1) elucidate the contributions of Scn5a to cardiac pathophysiology in DM1, (2) reveal a potential therapeutic target for cardiac abnormalities in DM1, (3), advance our current understanding of the cardiac sodium channel splice variants and their functional importance and (4) provide a foundation for a better understanding of the molecular mechanism of arrhythmias and conduction defects.